1. Field of the Invention
The present invention relates to an FET band amplifier used in various receivers and so on.
2. Description of the Related Art
Band amplifiers that amplify signals of a predetermined band are used in various kinds of receivers such as an AM receiver and an FM receiver. Typical band amplifiers include an intermediate frequency amplifier that amplifies intermediate frequency signals. In the intermediate frequency amplifier, only signals of a narrow band close to the intermediate frequency are selectively amplified. The center frequency in this case is set to a fixed value such that for example, the center frequency is 10.7 MHz in the case of the FM receiver, and the center frequency is 455 kHz in the case of the AM receiver. Furthermore, generally, an automatic gain control circuit is connected to the intermediate frequency amplifier for setting an optimum gain according to the level of electric field strength. For example, in the AM receiver, there is provided an automatic gain control circuit that controls the gain of the intermediate frequency amplifier to be kept at an appropriate value according to the output level of an AM detection circuit.
A multistage amplifier with transistors connected in multiple stages is generally used for obtaining a predetermined gain in the band amplifier. At this time, if a noise occurring in the transistor of each stage is large, the noise component is amplified and accumulated in the transistor of each stage, and therefore the noise component included in a signal outputted from the transistor of the final stage is increased. If the noise component occurring in the band amplifier itself is increased in this way, a problem arises such that a residual noise is increased when the electric field strength is high and the gain of the band amplifier is controlled to be kept at a small value. Especially, if the band amplifier is formed using a CMOS process, a MOS-type FET is used as an amplification element. Generally, the MOS-type FET has a large amount of 1/f noise appearing in a low-frequency range compared to the bipolar transistor, and therefore requires some countermeasures.